In zeolites, the locus of the catalytic activity is associated with framework aluminum. However, the amount needed for some catalytic applications is lower than that achieved by direct synthesis.
It is customary, in some cases, to produce a zeolite with a reduced framework aluminum content and to subject the zeolite after synthesis and calcination to a treatment with steam. That treatment progressively hydrolyzes some of the framework aluminum, rendering that portion non-catalytic, until only a desired amount of catalytic framework aluminum remains. This can require severe steaming conditions. In some cases, such steaming can damage other portions of the catalyst composition being prepared. For example, if the zeolite also has been provided with a metal component, damage can occur to that component. Also, progressive steam treatment must be terminated at just the correct time and temperature conditions, because further removal of aluminum will proceed.
Thus, it is desirable to have a procedure which more sharply defines the exact point at which removal will no longer proceed, that is, a procedure in which the remaining aluminum concentration will be sharply defined. It is the purpose of this invention to provide such a procedure.
In the past, the production of a low aluminum zeolite, for example, from Zeolite Beta, has been performed by synthesis to contain a desired amount of aluminum, but an excess amount of boron, for facilitating crystal growth. However, it is found that the boron remaining in the zeolite facilitates subsequent substitution by additional aluminum by migration from alumina binder. Binder is commonly used to formulate final catalyst mass into pellets, particles or extrudates. According to the present invention, excess boron would be substituted by silicon atoms, leading to stabilization of the crystal structure.
The use of a silicon halide, such as silicon tetrachloride, to perform a substitution of part of the aluminum existing in a zeolite has been taught generally by U.S. Pat. No. 4,273,753, hereby incorporated by reference. The present invention provides more specific methodology to perform a controlled and selected amount of substitution by silicon.
Thus, it is the purpose of this invention to overcome the disadvantages of the prior art and obtain zeolites having a high Si/Al ratio. It is a further objective of the present invention to produce a zeolite having high activity that has a stable dealuminated structure.